Antibodies to the class 2/3 OMPs (PorB) are bactericidal and antigenic diversity between these proteins forms the basis of the current serotyping classification. We have examined the variable regions of the porB gene from the 13 serotype strains expressing class 3 OMP, and have developed a panel of oligonucleotide probes for identification and classification of strains using genetic techniques. We have used this system of molecular analysis in several ongoing projects: 1) Molecular epidemiology of meningococcal meningitis in Brazil using PCR amplification of the porB gene directly from cerebrospinal fluid and subsequent genotyping using oligonucleotide probe hybridization. 2) Examination of serotype 15 strains obtained from cases of meningococcal disease among vaccine recipients and controls in the Norwegian outer membrane protein vaccine trial. 3) Epitope mapping of serotype 4 and serotype 21 monoclonal antibodies by comparison of antibody binding and variable region probe hybridization among a variety of type 4 and type 21 strains. 4) Investigation of an unusual strain sent to us from the CDC which binds both type 4 and type 15 Mabs. Using both probe hybridization and direct sequencing, we identified which regions of the porB gene from this strain were similar to each of the two prototype serotype strains We are currently investigating the antigenic properties of the class 3 OMP from this strain in rabbit immunization studies.